1. Field of the Invention
This invention relates generally to gradient index ("GRIN") lenses, and specifically to a method for making, and the use of positive and negative Abbe number radial GRIN rod lenses as optical relays in endoscopes, borescopes, and similar type instruments referred to hereinbelow as "remote viewing scopes".
2. Description of the Related Art
Remote viewing scopes are well known in the art for their use in viewing, through relatively small openings, body cavities and industrial vessels, for example, where direct viewing is not possible. A typical early scope instrument is described by Hopkins in U.S. Pat. No. 3,257,902. It employs a series of rod lenses forming an objective system and a relay system for forming and relaying the image of an object down a narrow tube to a camera.
Systems of this type typically undercorrect axial chromatic aberration due to the convex, or positive, refracting surfaces of the lenses which are necessary to converge light and form real images. The aberration appears as a halo around the image because the blue light components of the image are focused closer to the lens than the red components along the optical axis. Ideally, an image formed in whim light would have a common focus for all wavelengths or colors and thus be a clear and true representation of the object being viewed.
Since classical optical elements having negative power, i.e. concave refracting surfaces, characteristically overcorrect axial color by focusing the red components closer to the lens than the blue components, axial chromatic aberration in the image formed by a Hopkins-type scope is correctable by appropriately using additional negative power lenses in the device. This approach, however, adds to complexity and cost, especially in view of the frequency of use and sterilization requirements of modem devices of this type. Leiner and Prescott, in Applied Optics 22, 3 p. 383 (1983) also point to difficulties in manufacturing classical lenses of the required size and surface contours for use in scope-type instruments.
Around 1970, Nippon Sheet Glass (NSG) developed solid gradient index rod lenses for use in scope-type optical systems. An advantage of the NSG GRIN rods was their flat refracting surfaces. In addition, it was realized that light rays propagating through the lens follow a sinusoidal path having a defined period when the index gradient profile varies essentially parabolically as a function of lens radius. The rod lens could then be cut to a design length corresponding to the periodicity of the light propagation path to achieve desired imaging results. Axial and radial gradient index lenses are now well known in the art. Moore et al. in U.S. Pat. No. 3,729,253, describe gradient index properties and cite references to the design and manufacture of GRIN rods used, for example, as image relays. The disadvantage of GRIN lenses produced by NSG and others, however, results from their characteristically high dispersion. High dispersion, whether positive or negative, does not contribute to good image quality which is valued in applications utilizing remote viewing scopes. Surprisingly, NSG and others who have made radial gradient lenses for years have not produced a negative radial GRIN dispersion material, or even very low positive dispersion despite the knowledge that positive dispersion is a major limitation in many imaging applications, particularly remote viewing scope relays.
An inherent problem of all conventional lenses is the chromatic aberration created by dispersion of the lens material itself. Optical dispersion is the change in the index of refraction as a function of the wavelength of the light passing through the lens. In general, the refractive index of a material is larger in shorter wavelengths than in longer wavelengths. Thus, the index of refraction in blue fight is greater than in red light. Positive dispersion, therefore, is responsible for undercorrected axial color in optical systems. Similarly, dispersion of a GRIN lens also contributes to axial chromatic aberration.
Thus the inventors recognize a need for GRIN components which characteristically overcorrect axial color, and which can be used to simplify the optical systems of remote viewing scopes for color corrected imaging. A remote viewing scope utilizing this invention is particularly useful, for example, in dental applications and has several advantages over presently available dental remote viewing scopes. The gradient index relay is lower in cost than conventional relay technologies and can be made in smaller diameters, allowing more room in a patient's mouth for illumination and dental work. In addition, a short focal length objective in connection with the gradient index relay gives increased depth of focus over conventional systems. Furthermore, the negative dispersion of the gradient index relay balances the positive axial aberration inherent in optical lenses with net positive power.
It is therefore an object of this invention to use negative dispersion GRIN lenses in the optical systems of remote viewing scopes to reduce or eliminate axial chromatic aberration in the image.
It is a further object of the present invention to use negative dispersion GRIN rods in integral and/or optical combination with conventional optical components in remote viewing scopes to reduce or eliminate spherical aberration and coma in addition to axial chromatic aberration in the image.
It is another object of this invention to use radial GRIN lenses in optical and/or integral combination with diffractive or holographic optical elements in remote viewing scopes for axial chromatic aberration correction.
It is a still further object of the present invention to use a plurality of radial GRIN rod lens segments or sections in a remote viewing scope to give such a device a degree of physical flexibility in addition to corrected imaging capability.
It is another object of the invention to produce remote viewing scopes having smaller diameters, greater depth of focus and better imaging qualities at reduced cost over remote viewing scopes having similar characteristics using conventional technology.
Finally, it is an object of this invention to provide a method of producing a negative dispersion radial gradient index relay.